The present invention relates to the protection of integrated circuits (ICs), and more particularly, to a scheme for preventing the reverse engineering of ICs. The invention is particularly useful in protecting secure ICs which are used in cable and satellite television decoders to prevent unauthorized users from receiving television broadcasts. The invention is equally useful in protecting secure ICs used in other applications, including terminals and smart cards for electronic funds transactions, premises access control, electronic games, and the like.
Due to the continuing popularity of the pay television market, there exists a tremendous financial motivation for unauthorized persons known as "pirates" to modify the access control of set-top boxes (e.g., decoders) to allow the reception of television programming without payment of the required subscription fees. The modified decoders are purchased by unscrupulous individuals through various markets and used to illegally receive and view the television signals.
To produce a modified decoder, a pirate must extract certain information from a genuine decoder which is usually known only to the authorized manufacturer. The decoder typically includes a secure (e.g., cryptographic) processor which contains information such as cryptographic keys which are used in descrambling a scrambled television signal or other programming service signal (e.g., audio or data). Since the secure processor performs access control functions, it is a focal point of the pirate's attention. Accordingly, the pirate will employ various techniques in an attempt to gain information from the secure processor.
One common attack technique is known as "probing." A secure processor includes an integrated circuit (IC) which is fabricated as a monolithic device with an ensemble of active and passive components, including transistors, resistors, capacitors, and diodes which are interconnected within a monolithic block of semiconductor material. During probing, ICs such as very large scale integrated (VLSI) circuits are subject to an invasive attack wherein the die (e.g., IC or "chip") is exposed by decapsulation. During decapsulation, the compound material which encapsulates or surrounds the die is systematically removed. Then, probes which measure current and other parameters are used to monitor the electronic signals in the active components of the chip. The term "tampering" as used herein is meant to encompass both probing and decapsulation.
A pirate may perform the following decapsulation steps to prepare a chip for probing. First, the chip is removed from the decoder board, with the die still inside the IC package. Generally, this is the case when a chip is mounted on a large board. The decoder board may be a computer board such as those used in a personal computer (PC). If the chip requires a direct current from a battery or the like to circumvent a self-destruct feature, then battery wires are soldered to a positive voltage pin (e.g., V.sub.batt) pin and to a negative voltage pin (e.g., V.sub.ss) on the outside of the chip prior to removal from the board. The chip is then removed from the board with the battery wires still attached. If the battery power is interrupted, the chip may self-destruct by erasing critical information stored in memory. The pirate can identify the appropriate battery pins by taking resistance measurements of the traces on the board which are connected to the battery, and then confirming by taking voltage readings of the traces with a main voltage (e.g., V.sub.cc) off.
Second, the location of the die within the encapsulating compound of the IC package can be determined by taking an x-ray of the IC package. Third, a mechanical grinder can be used to remove as much of the encapsulating compound as possible above the top surface of the die without damaging the die. Fourth, chemical etching or plasma etching is performed to remove the last portions of encapsulating compound which remain over the area of the die which is to be probed. Some chemical etchants work so well on the encapsulating compound that the grinding step can often be skipped.
In current chip designs, including those for application specific ICs (ASICs), a pirate will generally not encounter significant obstacles in performing the four steps above. Removal of the chip from the board with the battery wires still attached is usually considered to be the most delicate operation. Thus, the undamaged die, which is protected by an inert glassivation coating, can be exposed by a pirate as long as a short circuit or open circuit of the battery power is not created. Furthermore, damage to bond wires can also easily be avoided. Bond wires may connect bond pads of the chip to package pads in the protective encapsulating package, and are located on the periphery of the device, as will be discussed in greater detail below. The decapsulation process can avoid exposing the die in the area where critical bond pads are located, that is, where the battery power is input into the active component.
One approach to deterring probing is discussed in commonly-assigned U.S. Pat. No. 4,933,898, issued Jun. 12, 1990 to Gilberg et al., entitled "Secure Integrated Circuit Chip With Conductive Shield." Gilberg et al. disclose using one or more conductive layers to overlay a secure area of an IC. The conductive layers shield the secure area from inspection and carry a power signal to the IC. Removal of one of the layers by a pirate causes the loss of power to the components of the secure area. However, the implementation of this approach is somewhat complex.
Accordingly, it would be desirable to provide an apparatus which deters or otherwise hinders probing of an IC chip by a pirate. In particular, it would be desirable to provide a barrier to decapsulation which uses standard bond wires to render the IC non-functional when the bond wire is ruptured. The barrier should be compatible with existing chip designs and inexpensive to implement. The present invention provides a system having the above and other advantages.